This invention relates to methods and systems for continuously monitoring emissions of fluids, particularly gases, in flue stacks and other ducts found in industrial plants and electric utility power generating Stations, and other industrial installations.
With the increasing concern and attention directed to the nature and quantity of industrial emissions, the need has arisen for more accurate techniques for measuring emissions from flue stacks and ducts. One of the major driving forces in this current interest in measuring emissions is the development of strict regulatory procedures at several governmental levels setting minimum standards for acceptable operation of a utilities plant and other industrial plant operations, and providing heavy penalties and other sanctions for violating these standards. One of the major problems encountered in the implementation of the emission standard programs is the lack of proven and reliable systems for measuring the actual values of certain key parameters. While theoretical and pilot systems have been developed to measure many of the key parameters, the implementation of these systems has been found to be very difficult given the extremely hostile environment of industrial flue stacks.
One of the key parameters whose measurement with relative precision is required for reliable continuous emission monitoring is the flue gas volumetric flow rate. In the past, attempts to design and implement measuring systems for determining the volumetric flow rate in industrial stacks have focused on three different types of measuring techniques: thermal dispersion, differential pressure sensors, and ultrasonic sound generators. The thermal dispersion technique employs sensors using "hot wire" or "hot film" cooling to provide an electrical signal representative of fluid temperature. The principle of operation of these devices is based on the phenomenon that fluids passing a heated sensor cool the sensor at a rate proportional to the fluid velocity. In operation, the sensor is inserted into the gas flow path and heated to a constant temperature higher than the temperature of the gas flow being measured. The electrical power required to maintain constant temperature in the velocity sensor is in part proportional to the convective cooling effects of the measured gas flow, which permits the actual velocity to be inferred. Sensors of this type suffer from the disadvantage of becoming readily contaminated with materials present in the gas stream, which requires frequent maintenance in order to maintain the sensors reasonably accurate. Also, the presence of moisture in the gases flowing past the sensor also corrupts the accuracy of the measurement, since most fluids remove sensor heat at a rate which is dependent more on the percentage of moisture in the fluid than on the rate of flow of the fluid.
Differential pressure sensors operate by extracting flow rate data from flowing fluids as a differential relationship between the upstream and downstream pressures. The typical differential pressure sensor employs one or more pitot tubes or a device known as an annubar. These sensors suffer from the disadvantage of requiring bulky, cumbersome arrays of primary measuring instruments which are subject to plugging due to particle contaminants in the gas flow. In addition, the calibration of such devices to the required accuracy is, at best, difficult to achieve and maintain.
Ultrasonic systems use the flight time of sonic pulses between two locations along the bounded fluid path of the flue stack to determine the path average velocity of the gaseous medium U.sub.p. This technique suffers from the disadvantage that the path average velocity of the gaseous medium U.sub.p is only an approximation of the actual average volumetric flow rate U.sub.m and only accurately indicates U.sub.m when the velocity profile is uniform over the entire cross-section of the stack. However, due to the effects of viscosity in the regions adjacent the stack walls, the velocity profile is never uniform over the entire cross-section, and thus the sonic measurement technique is inherently inaccurate. It can be shown that an ideal, fully developed turbulent flow in a circular stack that varies approximately in accordance with Prandtl's law, which corresponds to a nearly plug flow condition, results in an error of 7 percent in computing U.sub.m using the known sonic technique. In addition to this inherent inaccuracy, the ultrasonic technique suffers from additional disadvantages. For example, in order to improve the signal to noise ratio between the transceivers required in the ultrasonic system, the transceivers must face each other at an angle to the stack wall. As a result, the lower (upstream) transceiver is exposed to both moisture and contaminants that run down the stack wall and enter the transceiver unit. While a purge air system can be used to reduce the exposure of the lower transducer to moisture and contaminants, care must be taken to insure that the introduction of this extra flow of air into the duct system does not affect the operation or sensitivity of the transceivers. Further, the ultrasonic approach is also limited by the requirement that the instrument be located far enough from the duct inlet so that the velocity profile may be assumed to be essentially symmetric and well developed (i.e., relatively non-turbulent). In practice, this usually requires that the sensors be located at a distance of at least 20 to 25 stack diameters from the flow inlet, which creates installation and maintenance problems. Efforts to eliminate or substantially reduce the drawbacks noted above in the thermal, differential pressure and acoustic ultrasonic systems have not met with success to date.